The CWNA book mentions that you should not stand in front of a highly directional antenna while powered on. I'm wondering if the same idea is true for omnidirectional APs. Is there a safe distance you should maintain from an AP? For example, I need to mount an AP in a lobby and the easiest place at this time is under the receptionist desk. I want to know if it is dangerous to install it within a certain distance of the person sitting at the desk.

I think I've read that about 18 to 24 inches is the closest you should be to an AP. Personally I try to keep at least 5 feet away, but I run a lot of strange tests so they are often closer than that. I don't install AP's closer than 10 feet from anyone else.

From a performance standpoint, the last place you want to put an AP is next to the floor. The minimum height I would place an AP at in a home environment would be 5 feet, and 8 feet in a business.

Too high a placement causes problems too. In that case we start looking at patch antennas or down-tilt.

Radio waves from antennas, mounted too close to the ground, are reflected mostly upwards. In addition, the desk and receptionist will be absorbing some of the signal, and you would have to raise the power level to try to overcome the losses. In todays lawyer-infested climate you would also be asking for lawsuits for any medical problems, whether real or imagined.

I have a downtilt omni AP that I intended on reverse mounting somewhere under the desk so I'm pointing the majority of the signal upwards. The AP would be roughly 3 feet above the ground. I'm covering a small space so I'm not too concerned with attenuation. I planned on surveying the AP in different positions/places to make sure I get the level of coverage needed. This may not turn out to work at all, but I have limited options at this time and thought it'd be worth a try.

Any chance you recall where you've heard the recommendation of AP distance between bodies? I'm just curious to know if there is a definitive source of this information since the CWNA book was somewhat unclear. I do appreciate your input and suggestions.

This is one of the most complex areas of discussion in radio systems in general. It is a divisive issue, that gets many people “hot under the collar”.

I’ve had over 25 years of experience in working with RF systems and will try to put down a few pieces of information, that may be useful.

Whenever I have tried to rationally explain some of these issues verbally, some parties just don’t want to listen and simply “switch off”. That is their perogative. However, this is a very important issue for discussion, and even though these are only my “five cents’ worth of” comments, I hope that even if people do not agree with any or all of what I write here, that it will provide material for further research.

We know from experience that different human beings can experience sensations differently.

For example, we are at a barbeque and John says “Whooooo-eee !! These chili barbeque wings are burning my mouth !!”

So, what has happened here ? In our mouths, there are “sensors” for both taste ( sweet, sour, salty, bitter ), as well as for “heat”. Chemical reactions take place and a sensation of “heat” is transmitted to the brain. These “sensors” have sensitivity levels that vary between human beings in a similar manner to the RF sensitivity levels of WLAN cards made by different WIFI manufacturers. In fact, even between cards of the same model made by the same manufacturer, we can have slight differences.

A bunch of little kids are twisting each others arms to see who can last longest without crying out. Bobbie wins and all the kids say “Bobbie’s the strongest !! He can take the most pain !!”

Perhaps, perhaps not. The complex sensors that allow us to feel pain, have different thresholds, as per “the threshold of pain”. This is a very complicated business, due to electro-chemical reactions, muscle tension, etc.

The point is that as biological entities, we have various sensors that produce responses that vary between individuals. In other words, each person’s sensitivity level is different from one person to another ( sometimes dramatically so ).

In the next part, we’ll talk about two parts of the human body that can be affected by RF signals ( the degree to which they can be affected is probably the most controversial area ), the eyes and the brain.

Steve and Mike are down at the beach, and Steve says “I can’t believe you’re not wearing sunglasses…..I have to wear mine”. Mike shrugs and says “It doesn’t bother me…never has”.

When optical energy hits the front of the eye, it is focused onto the back of the eye, where sensors convert the brightness, colour and shading information etc into electrical signals that the brain then interprets in the form of a picture. A miraculous process. The pupil controls the amount of light that is let in.

If anyone has ever had their eyes dilated at the opticians, they may know how painful it can be to stand outside after treatment and not put on the little pair of sunglasses that are usually provided.

Different people have different sensitivities that affect how they perceive light. I will return to this issue of sensitivity later when we look at the question of “how far away from an AP do I need to be ?”. Or do we in fact need to bother about this at all ?

Light is a form of electromagnetic energy, as are radio waves. We know that a microwave oven is a very effective device for heating up food. We may also know from our CWNA studies that microwave ovens can be a potential source of interference for some of our Wi-Fi signals in the 2.4 GHz band.

Does this mean that our Wi-Fi signals are “cooking us ?”

The answer to that is more complex than it sounds, but there is no need to panic.

Firstly, microwave ovens are heating foodstuffs in a confined, close space, hopefully with a proper seal ( yes, if you have an old microwave oven with a loose door, it would be a good idea to get that door repaired or replaced ). A Magnetron or similar device, used within the oven pumps out quite a lot of power. These power levels are much, much higher than what a Wi-Fi system produces.

It is an established fact that radio waves of certain frequencies cause heating effects in biological tissues. The most sensitive organ in the human body as far dangerous effects from heating is concerned, is the eye.

The possible damage to the eye due to heating from radio waves depends upon a multitude of factors, including, but not limited to: frequency, amplitude ( or rather power density ), pulsed or non-pulsed etc.

Power density is an important factor, discussed under CWDP studies. Essentially, when we measure power density of a radio signal, we are measuring how much power per square meter or inch or yard, or whatever unit we care to use, exists.

If anyone is familiar with “Maglite” or similar flashlights, they can be used to demonstrate this. The Maglite ( which every Wi-Fi engineer should carry in his or her toolkit for peeking behind ceiling panels prior to installing an AP etc ) has a focusing lens. The device always sends out the same amount of power ( ignoring battery ageing ), but the amount of power per square meter can be adjusted via a focusing lens. These things are really powerful. If you shine the beam with the focus opened up to give a broad beam in a dark room, you could move the beam around, and someone else in the room would just cover their eyes if it him them. However, when we narrow the focus down, a very bright beam indeed is produced, and I would not advise shining it into anyone’s eyes. The power density ( or power flux density ) is very high in the latter case.

The heating effect on the eye depends not so much on the total power radiated, but by the power flux density.

I’ll talk more about this and the effects on the brain in the next part.

The brain is the most complex organ in the human body. Seemingly countless numbers of neural pathways wind their way through it’s structure. The brain is highly susceptible to temperature changes. This is known from cases of extreme heatstroke where permanent damage has occurred. We know that RF signals easily pass through the human skull on one side, through the brain and eyes and out the other side.

The largest area of contention at present regards the long-term effects of RF energy on the brain. Trying to get accurate data is almost impossible, as people seem to be aligned in one of two camps: “Ain’t nothing wrong with them radio signals….my grandpappy worked with radios for years, and he’s as right as rain”….or…..”It’s the end of the world !! Those Wi-Fi signals are killing us….head for the hills !!”

This is very similar to the situation regarding climate change, where both sides have strong views, and there have been cases of data having been manipulated by both sides to promote their respective views.

Laboratory tests have been done on mice, which have shown tumour growth due to long-term exposure to certain RF signals. There are multiple problems here. Firstly, mouse biology does not translate directly to human biology. Next, the mice were “held captive” in one place with a constant bombardment of RF, etc.

We, as humans, do not undergo this constant bombardment to the degree that the lab mice did.

However, does this mean that we should not be concerned with the effects of RF ?

As a way of getting to that, let us examine a couple of other health related issues.

Nowadays, just about everyone knows that smoking is not good for you. In the early 1950’s, however, it was actually promoted as being healthy for you. You can Google this and see old ads showing doctors recommending brand “X” to their patients.

As more data became available and the cigarette companies were forced to admit additions of addictive compounds, etc, the public became more aware of the health risks. Did this mean that overnight, everybody suddenly decided to stop smoking ? No, it did not. Many people continue to smoke. I am pretty sure they are aware of the dangers, but continue to do so. This is a personal choice. Areas have been designated for smokers. In the past, in restaurants and airliners, non-smokers were forced to inhale air which contained smoke. Nowadays, there are many more safeguards in place to keep non-smokers away from smoke.

What about people who say “My uncle smoked four packs a day and never got cancer ?”

We know that some people are genetically predisposed to being able to fight certain types of developing cancers. They are rare, but do exist. Statistically, we cannot say that just because a few people are able to smoke all their lives with no apparent ill effects, that smoking is harmless. We, instead have to look at a broad range of statistical data.

Have you ever been to a major beach and seen it completely empty in the summer ? Very rare, I am sure. People know that over exposure to UV rays can be dangerous, yet at the same time, we know that sunlight has health benefits. It is a matter of choice.

We take risks every day. Crossing the road, driving a car, etc. However, these acts are part of our daily lives. We accept that we have to take some form of risk each and every day just to exist.

If we wanted to, we could live in houses all the time and never go out. That way we would not be exposed to second hand smoke and UV rays. However, that is impractical.

In the next part, I’ll discuss some personal observations that I have made over the years regarding RF exposure.

I worked for many years on large satellite stations. These stations carried international communications between major countries. The antennas were over a hundred feet across and the whole assembly of antenna plus backing structure weighed over three hundred tons. The antennas were parabolic in shape with a hyperbolic sub-reflector. The main “dish” was made up of hundreds of individual panels fitted close to each other, but with small gaps to allow for thermal expansion etc. The panels all attached via adjusting screws to a “backing structure” which acted as a support for the panels. When you built one of these, you had to drive the dish until it was pointed upwards. You then went inside the dish ( at night so that there were equal thermal characteristics across the entire antenna….during the day, some parts would be in shadow and others illuminated by the Sun ). You also had to have the antenna pointed upwards, as when the antenna is pointing away from vertical, there are different gravitational forces on the panels at the bottom compared to panels at the top. A theodolite would be used with panel markers to see which panels needed to be adjusted in order to keep the antenna shape perfectly parabolic. Nowadays, laser rangefinders are used. Riggers would be “hanging off” the back of the backing structure and using radios, would adjust the panels.

Once the antenna was operational, maintenance procedures would be carried out, whereby riggers would remove rust from the metal parts and perform painting. This would be done while the antennas were operational and transmitting energy. Tiny amounts of the RF energy would leak out of the dish and pass through the small gaps to whoever was behind the antenna.

The riggers were highly experienced and had been doing this work for years. Out of that small group that I knew, one went blind in one eye and had the eye removed. One developed cancer of the jaw, another cancer of the throat and yet another developed another tumour. I can tell you that statistically, the chances of this happening by chance to a little group like that are practically zero. The chap who lost his eye was asked by his doctor “Do you work around radio waves ?”

I had mentioned this in a post on this forum several years ago, and one person became highly indignant and said something like “Well, we’re not talking about big powerful satellite and radar signals, we’re talking about low power wi-fi. It’s completely different” Unfortunately, he hadn’t grasped what I was trying to say.

You would imagine that satellite stations send out thousands of Watts of physical power. In actual fact, many send out only a few Watts of power. They use the very large gains of the parabolic reflectors to get a suitable EIRP to overcome the various losses involved in sending a signal 22,000 odd miles into space to where the geostationary satellite is.

That small amount of power was then “spread” over the entire surface area of the parabolic antenna. This meant that the Power Flux Density which we spoke about earlier was very, very low indeed. That meant that only a tiny, tiny amount of energy was leaking out through the panel gaps. Not the vast amounts that he may have imagined.

What about frequencies ? Those antennas used C-Band communications, 5.925 – 6.425 GHz, very, very close to conventional 5 GHz Wi-Fi frequencies.

The point here is that those riggers were exposed to those frequencies and associated power levels over a long period of time, and that is very important. RF biological damage is cumulative.

In the next section, I’ll wrap up these notes by discussing AP location and cellphone issues.